What is critical to check before coupling a tractor to a trailer?

Answer: B — The air supply system for leaks

Checking the air supply system for leaks before coupling a tractor to a trailer is crucial because the air brake system relies on a constant supply of air pressure to function effectively. If there are any leaks in the air supply lines, it can lead to insufficient brake pressure, which compromises the ability to stop the vehicle safely. A leak could prevent the trailer brakes from engaging, creating a significant risk during operation, especially in emergency situations. Understanding the air supply system and ensuring there are no leaks helps maintain the overall integrity and functionality of the braking system, which is essential for safe vehicle operation. Additionally, it is part of a pre-trip inspection protocol that ensures the equipment is in proper working order before the journey begins. Regularly checking for air leaks promotes safety and compliance with training and regulations for commercial drivers.